Method of preparing polymeric solutions



rmmd is, 2a, 1846 TheronG.I-'insel,Cleveland,0hio,alllgnortoE.L du Pontde Nemonn Company, Wilmington, Del, a corporation of, Delaware 7 IDrawing No Continuation No. 491,943; June 23. 1 Mayll, 1945. No." 593.31

1 (crew-1st) This invention relates'to the preparation of solutions ofpolymeric materials, and more particularly to the dissolvingof polymersof acrylonitrlle toform clear and lutlovns thereof. This applicationisacontinuation of my copending application Serial No. 491,943, flied June23, 1943. I I

Copending application of Ray Clyde Houtz,

Serial No. 447,446, illed June 17,4942, and

pending application of George Henry Latham, Serial No. 447,466, fliedJune 17, 1942', describe procedures for dissolving acrylonltrilepolymers in volatile organic solvents to form solutions which arecapable of being extruded through suitable apparatus to form shapedarticles, such as films, filaments and the like. Organic solventsolutions of acrylonitrile polymers prepared as described in saidcopending applications show no tendency for the polymers to separate outfrom solution at ordinary or low temperatures for example roomtemperature (75 F.) or lower, but it is dimcult to initially preparesuch solutions at ordinary temperatures of the order of 80 F. or less bymerely stirring the polymer with the solvent because of the tendency ofthe polymer particles to become swollen and covered with a viscouscoating which hinders the dissolving action of the solvent, the polymerparticles tending to aggregate or 'ball. This balling tendency can beovercome with dissolving of the polymer by stirring the mixture ofpolymer and solvent vigorously forextended periods of time at elevatedtemperatures, but the resulting solutions are highly colored and aretherefore not completely satisfactory for the production of shapedarticles, such as filaments and films. The initial appearance of colorcan be somewhat retarded by stirring the polymer and solvent in anoxygen-free atmosphere, but even with the use of this expedient, anundesirable amount of color develops before the polymer is completelydissolved.

Application of Ray Clyde Houtz, Serial No. 491,945, filed June 23, 1943,describes procedures preventing this undesirable balling of the polymerparticles. This is obtained according to the said Houtz application bytemporarily modifying the solvent to such an extent that it is no longercapable of dissolving or even substantially swelling the polymer, andstirring this modified solvent with the polymer particles (whichparticles have been ground to a particle size not larger substantiallycolorless sofanlioatlonsedal r 3. application Preferablyjobtalned by itto. a'low temperature, for example 03 CL, although it can also i beobtained by adding to thesolvent a miscible liquid that is a non-solventior'tlie, polymer, the "liquid preferably being low-boiling so that itwill later be removed fromthe solvent by heating.

Suitable liquids includegasoline, 'dimethyl ether and acetone.Subsequent rapid heating of the slurry: of polymer particles andmodified solvent removes the modifying agent and results in the smooth,rapid formationjof adesirable, substanv tially colorless solution of thepolymer.

Such a process operates to yield a solution that is eminently suited foruse in the manufacture of shaped articles that are free fromobjectionable color. "However, th preferred method of operatlng theprocess, involving the step of cooling the solvent, requires the use of.large, relatively extensive cooling equipment, while the other proposedmethod involves the use of considerable amounts of relatively expensivechemicals, whose recovery adds further wan s-0st of'op'eration.

It is an object of this inventionto provide an improved method forforming substantially colorless (water white or very li ht amber)solutions of polymeric materials whichcanb dissolved only withdifliculty and the solutions of which are sensitive to heat and becomecolored upon pro longed exposure to elevated temperatures. A specificobject of the invention is a method for forming a substantiallycolorless solution of an acrylonitrile polymer dissolved in a suitableorganic volatile solvent. Other and additional obiects will becomeapparent from the description hereinafter set forth.

The objects of the invention are accomplished, in general, by mixing asubstantially colorless polymer, which normally develops color orprolonged exposure at elevated temperatures, i. e. C. to C. and higher,in a finely divided state in a solvent having a gaseous acidic anhydridedissolved therein. The polymer-modified solvent mixture is rapidlystirred and converted into a uniform slurry with substantially notendency of the polymer to ball. The slurry is then heated at atemperature of 100 to 150 C. for a period of time to permit evolution ofthe gaseous acidic anhydride from the solvent and the solvent toexercise its strong dissolving effect on the polymer and convert it to ahomogeneous, substantially colorless solution.

all acidic anhydrides that exist as gases at 20 C. and atmosphericpressure, such as, for exam- 3 sulfur dioxide, the various oxides ofnitrogen and the like. a I

By the term volatile organic solvent as used throughout thespecification and claims is meant an organic solvent which can beremoved sub- 5 stantially, completely by evaporation from a solutionprepared therewith.

Example I Sulfur dioxide gas bubbled into flasks, each of whichcontained approximately 41 parts of dimethyl iormamidaat roomtemperature, to form solutions containing 3.5%, 7.8% and 19% sulfurdioxide. 9 parts of finely divided acrylonitrile polymer having amolecular weight of 130,000 and previouslyground to a particle size orapproximately 200 mesh were then added to each of the flasks at'roomtemperature, and the mixtures rapidly stirred until an opaque, uniformsuspension of the polymer particles. in the liquid was formed. Thesuspension formed from the solution containing 3.5% sulfur dioxidebecamerather viscous after standing for 2 hours at room temperature. However,at the end 0120 hours at room temperature. it could still be stirred andshowed no tendency to separate or form into difflcultly soluble balls orlumps. Aportion of the slurry was heated at 100 C. on a water bath. Aslow stream of S0: was given off and the slurry showed signs of forminga solution at the end of 30 minutes. The slurry was completely convertedto a substantially colorless solution at the end of 3 hours. Continuedheating of this solution for a total of 16 hours did not cause it to,color or darken appreciably. Another portion of th initial slurry washeated in an oil bath at a temperature of 120 C. to 130 C. The slurryformed into a substantially colorless solution at v the end of minutesof heating. Continued heating at this temperature resulted in only aslight increase in color of the solution.

The suspension formed from the solution containing 7.8% sulfur dioxideflowed freely after, standing for 3 hours at room temperature. It becamesomewhat more viscous but could .still be poured after standing for aperiod of 20 hours. When heated on a water bath at a temperature of 100C., the slurry formed slowly into a substantially colorless solution.This solution was formed much more rapidly by heating at 115 C.Continued heatingof the solution for a total of 16 hours at 100 C.caused only a slight increase in the color of the solution.

The suspension formed from the solution containing 19% sulfur dioxideflowed freely at the end of 20 hours. When heated for 15 to 20 min-.utes in an oil bath at 120 C. to 130C" B02 was evolved and asubstantially colorless solution of the polymer was obtained.

In contrast to the above procedure, .9 parts of 5 the same finelydivided acrylonitrile polymer were added with stirring to 41 parts ofdimethyl formamide containing no sulfur dioxide. The mixture was allowedto stand at room temperature and within 10 minutes formed into atranslucent, golden mass that contained visible balls or lumps. Whenheated on a water bath at 100 C. or in an oil bathat a temperature of120 C. to 130 C., the balls dissolved slowly, a'clear so; lution beingobtained at the end of 3 hours. The 75 4 solution, however, was reddishorange to brown in color. Continued heating of the solution at 100 C.resulted in further appreciable discolors: tion. Yarns spun from thissolution possessed an undesirable dark color. on the other hand, yarnsspun from the solutions pr pared with the use of were substantiallycolorless.

Example ll 10 parts of solid carbon dioxide (dry ice) were stirred with9 parts of the finely divided acrylonitrile polymer of Example I. Aftera fewminutes, during which time the air was eliminated from the mass ofpolymer particles, approximately 40 parts of dimethyl formamide wereadded to the mixture of polymer particles and solid C02, the CO:particles dissolving rapidly in the dimethyl formamide and the polymerparticles beingsuspended in theresulting solution in the form of auniform slurry. The viscosity of the slurry increased slightly onstanding at room temperature for 2 hours. When heated on a water bath at0., there was a g adual evolution of C0: and the slurry formed within aperiod oi approximately 1 hour to a clear, substantially colorlesssolution. Continued heating ofthis solution at this temperatureforatotal of 16 hours caused only a very.slight increase in its color.Yams spun from this solution were substantially colorless.

Example III Sulfur dioxide gas was passed into tetramethylene cyclicsulfone at room temperature to form a 27% solution. 60 parts of thissolution were mixed at room temperature with 8 parts of the finelydivided acrylonitrile polymer of Example I, and the mixture rapidlystirred. An opaque, uniform suspension of the polymer was obtained,thesuspension showing no tendency to separate out or gel and becomeviscous on standing at room temperature. When heated to a temperatureC., S0: gas was slowly evolved, and a clear, substantially colorlesssolution of the polymer was obtained at the end of 2 hours.

The aorylonitrile polymer treated in accordance with this invention ispreferably prepared by the ammonium persulfate catalyzed polymerizationof monomeric acrylonitrile dissolved or emulsified in water.

It can, however, be prepared by any other suitable type ofpolymerization reaction, such as, for example, the emulsiontype reactiondisclosed by United States Patent No. 2,160,054 to Bauer et al. Thepolymer prefer- Molecular weight= wherein:

' viscosit of solution viscosity of solvent 1 and C'=concentration ofthe solution expressed as the number of molesof the monomer'(calculated) per liter of solution.

The molecular weight of the polymer obtained is dependent on suchfactors as the concentration of the monomerin the water, the amount andtype of catalyst present, the temperature of the reaction, etc. When themonomer ispresent in 5% aqueous solution maintained at a temperature offrom 3' C. to 5 0., it is found that the use of 4% of timmoniumpersulfate catalyst (based on the weight of the acrylonitrile) resultsin the formation of a polymer having a molecular weight (as calculatedby the above equation) of approximately 60,000. Increasing or decreasingthe amount of the catalyst, while maintaining the other conditionsconstant, decreases or increases the molecular weight of the polymer.

Although the invention is particularly concerned with the treatment ofsimple polymers of acrylonitrile, it is to be understood that the in-'vention can be utilized to produce satisfactory solutions of otherpolymers of acrylonitrile which can be dissolved only in a limitednumber of solvents and at a temperature of 100 C. and above. Suchpolymers tend to develop color when subjected to prolonged heating insolution. Examples of such polymers other than the simple acrylonitrilepolymers are copolymers or interpolymers of acrylonitrile containing atleast 85% by weight of combined acrylonitrile. Thus, the inventioncontemplates within its scope the treatment of acrylonitrile polymerwhich has been interpolymerized with polymerizable substances, such asvinyl acetate, vinyl chloride, acrylic acid, its esters and homologues,styrene, isobutylene and other aces-res 6' acidic anhydrides react inwhole or part with the solvent or the polymer to form some complex orother compounds or dissolve orbecome entrapped in the solvent or becomeadsorbed by the polymer.

polymerizable substances; copolymers produced by the copolymerization ofacrylonitrile monomer with such other polymerizable substances are alsoincluded.

The method of this invention comprises the steps of forming a slurry ora suspension of a finely divided polymer in a solvent in the presence ofa gaseous acidic anhydride, and heating the slurry or suspension to forma clear, substantially colorless solution of the polymer. In general,the method of this invention is applicable to the preparation ofsolutions of any given polymeric substance in a suitable solvent.However, it is especially suited for use with those polymers that tendto darken when their solutions are maintained at elevated temperaturesfor prolonged periods of time. Typical polymers of this nature includepolymers prepared wholly or in part from monomeric vinyl or acryliccompounds other than acrylonitrile.

Any of the volatile. organic solvents utilized in the said copendingapplications of Houtz and Latham may be used, and these solvents includedimethyl formamide, dimethyl methoxy-acetamide, N-formyl morpholine',N-formyl' hexamethylene imine, butadiene cyclic sulfone, tetramethylenecyclic sulfone, p-phenylene diamine, and the mand p-nitrophenols, whichhave particularly marked dissolving power for the polymer at elevatedtemperatures.

The gaseous acidic anhydrides greatly reduce and modify the swelling anddissolving power of the solvent on the polymer so that it is possible toform at room temperature a stable slurry or The terms in the presenceof. "containinl." incorporated, dissolved" and the like, used in thisspecification and in the claims hereof, are intended to cover whatevertakes place upon the modification of the dissolving characteristics ofthe solvent by the gaseous anhydrides. I As indicated by the examples,it is preferred to use such gaseous acidicanhydrides as carbon dioxideand sulfur dioxide. However, other such materials, including the oxidesof nitrogen, can also be used. These acidic anhydrides are preferablydissolved in the solvent prior to the addition of the p lymer andformation of the slurry. However, they can, if desired, be added to thesolvent at the same time as the polymerparticles. The anhydride can beadded in the form of a gas or in a liquid or solid state; for example,carbon dioxide is much more easily handled as a solid rather than as agas.

The acidic anhydride must, of course. be present in the solvent to asumciently great extent to render the solvent substantially incapable ofswelling or dissolving the acrylonitrile polymer particles. The ease offormation and stability of the slurry vary directly with theconcentration of the acidic anhydride and, while it is preferred thatthe material be present in the solvent to the extent of at least 3% ofthe total weight of solvent and acidic anhydride, satisfactory resultscan be obtained with concentrations as low as particularly when theslurry is to be formed at room temperature or below. This is because ofthe increased tendency of the unmodified solvent to swell the polymerparticles and cause them to ball or lump together at elevatedtemperatures. Of course, higher concentrations of the acidic anhydrideup to and including saturation of the solvent can also be used.

. The polymer particles for use in the method of this invention arepreferably ground to a size of about 100 to 200 mesh or even smaller.Particles of such size can be very readily dispersed in the modifiedsolvent to yield a desirableslurry or suspension. This is not extremely.critical to the dispersion of the polymer in the solvent. Theyinvention, however, and satisfactory results can also be obtained whenlarger particles, the size of which does not exceed 20 mesh, are used.However, when the particle size exceeds 20 mesh, some difllculty isoften experienced in obtaining the desired slurry,

The temperature and duration of heating required to remove the acidicanhydrides from the slurry and cause the formation of av clear solutionof the polymer vary somewhat with the composition of the solvent and thenature and concentration of the acidic anhydride employed; for example,somewhat more strenuous heating is generally required in the case ofsolvent moditied with sulfur dioxide rather than with carbon dioxide.Similarly, the higher the concentration of dissolved acidic anhydride,the more strenuous must be the heating conditions. The exact conditionrequired for any given combination of solvent and acidic anhydride can,of course, be readily determined by simple experiment.

The polymer solutions obtained by the practice of this invention are notonly substantially colorless at the time, of their formation, but arealso capable of withstanding prolonged exposure to high temperatureswithout undergoing any further marked increase in color. This isevihydride. The exact concentration of the solution to be 7 dentlybecause the slow evolution of the gaseous acidic anhydrides of theinvention from the said solvent carbon dioxide.

slurry during the heating operation tend to sweep out any occluded ordissolved gases, such as oxygen, that might exert adeleterious eifect onthe polymer. Moreover,' because the gaseous acidic anhydrides of theinvention are relatively dense, they tend during the heating operationto form used in the formation, by extrusion, of shaped articles willdepend on the type of the shaped article to be formed and extrusionapparatus employed; Conventional apparatus generally requires that thesolution contain from to olymer by weight, and the process of thisinvention is admirably suited for the preparation of such solutions in acolorless state. The process is not, however, limited to the preparationof solutions of such concentration. It can be usedto advantage in thepreparation of polymeric solutions of almost any given concentration.

In the claims, the term slurry isintended to also include a "suspension.

. This invention provides a method for the preparation of clear,substantially colorless solutions of polymeric materials, such aspolymers of acrylonitrile that tend to decompose upon exposuretoelevated temperatures in solutions. The

' method is simple of operation and does not require the use ofcomplicated apparatus or of expensive chemicals, whose recovery isessential to the economical operation of the process. The

' method is well adapted for use on a commercial 'scale.

Since it is obvious that many changes and modifications can be made inthe above described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited thereto except as set forth in the appended claims.

I I claim: v

a 1. The process of forming clear and substane tially colorlesssolutions of acrylonitrile polymers which dissolve with difliculty andtend todevelop color upon heating or prolonged standing, which comprisesmixing such polymer in a finely divided state with a solvent for thepolymer, said solvent containing incorporated thereinxas an agent toreduce the solubility of the polymer in said solvent a gaseous acidicanhydride, to form a slurry, vand thereafter heating the slurry to atemperature. sumciently high to eifect solution of the I polymer whiledriving oi! the anhydride'inthe form of a gas, and maintaining suchelevated temperature until a homogeneous and clear solution is formed.

2. The process of forming clear and substan tially colorless solutionsof acrylonitrile polymers which dissolve with difliculty and tend todevelop color upon heating or prolonged standing, which comprises mixingsuch polymer in a finely.

divided state with a solvent forthe polymer, said solvent containingincorporated therein as an agent to reduce the solubility of the polymerin agent to reduce the solubility of the polymer in said solvent sulfurdioxide, to form a slurry. and thereafter heating the slurry to atemperature suiiiciently high to effect solution, of the polymer whiledriving off the sulfur dioxide in the form of a gas, and maintainingsuch elevated temperature until a homogeneous and clear solution. isformed.

4. The process of forming clear and substantially colorless solutions ofacrylonitrile polymers which dissolve with difiiculty and tend todevelop color upon heating or prolonged standing, which comprises mixingsuch polymer in a finely divided state with dimethyl formamide having agaseous acidic anhydrlde incorporated therein to form a slurry, andthereafter heating the slurry to a temperature sufiiciently high toeiiect solution of the polymer while driving off the anhydride in theform of a 8 .8, and maintaining such elevated temperature until ahomogeneous and clear solution is formed. I a

5. The process of forming clear and substantially colorless solution ofacrylonitrile polymers which dissolve with dimculty and tend to developcolor upon heating or prolonged standing, which comprises mixing suchpolymer in a finely. divided state with dimethyl formamide containingcarbon dioxide incorporated therein to form a slurry, and thereafterheating the slurry to a temperature suiiiciently high to effect solutionof the polymer while driving oil. the carbon dioxide in the form of agas, and maintaining suchelevated temperature until a homogeneous andclear solution is formed.

6."Th process of forming clearand substantiallycolorles solutions ofacrylonitrile polymers which dissolve with dimculty and tend to developcolor upon heating or prolonged standing, which comprises mixing suchpolymer in a finely divided state with dimethyl formamide containingsulfur dioxide incorporated therein. to form a slurry, and thereafterheating the slurry to 'a temperature sufiiciently high to effectsolution of the polymer while driving ofl. the sulfur dioxide in theform of a gas, and maintaining such elevated temperature until ahomogeneous and clear ..formation of a homogeneous and clear solution, awhile driving of! the anhydride in the form of THERON G. mm.

Patent No. 2,404,728.

' numberedpatent requiring correction as follows: Column 2,

Certificate of Correction j July 23, 1946; THERON G. FINZEL It isherebycertified that errors appear in the printed sfirlelcification oftheabove e 38, for or read on;

column 3, line 15, before bubbled insert the word was; and that the saidLetters 1 Patent should be read with these corrections therein that thesame may conform to the record of the case in the Patent Oflice.

Signed and sealed this 15th day of October, A. D. 1946.

' LESLIE FRAZER,

First Assistant Oqnwm'asioner of Patents.

